Recent advances in the development of semiconductor light-emitting diodes (LEDs) and organic light-emitting diodes (OLEDs) have made these devices suitable for use in general illumination applications, including architectural, entertainment, and roadway lighting, for example. As such, these devices are becoming increasingly competitive with incandescent, fluorescent, and high-intensity gas discharge lamps.
The amount of light emitted by an LED under operating conditions directly depends on the electronic current that flows through the device. Because of variations in device characteristics, the same device current can lead to perceivable different luminous flux output under certain operating conditions even when the LEDs are of the same kind. When the drive current of a series connection of LEDs is reduced below a certain limit, some LEDs can completely stop emitting light before others do which can result in an undesired mode of operation. Effects such as this are largely a result of fluctuations in forward voltage characteristics across different LEDs. The limits for tolerable luminous flux output variations across LEDs usually depend on the kind of lighting application. The LED characteristics typically should be closely matched when the lighting application requires more than one LED to be able to keep the drive circuit design as simple as possible. Several drive circuit designs have been suggested that can efficiently control the LED drive current. Most designs belong to a category of linear constant current drive circuits that can maintain a uniform drive current through a serial connection of LEDs. However, many linear constant current drive circuits incur high power losses and require special power electronic devices and diligent thermal management. Most direct current or switched current drive circuits require complicated feedback control systems to enable accurate and reliable luminous flux output control if a wide range of operating conditions including low fractional nominal light output is desired.
Other solutions to effectively control LEDs require a Buck-boost regulator generating a regulated common voltage supply, low-side ballast resistors to set the LED drive current, and shunt resistors for current monitoring. For example, U.S. Pat. No. 6,362,578 discloses how to use biasing resistors for current control and a voltage converter with a feedback loop to maintain a constant voltage across an array of LEDs. An additional transistor is connected on the low side of the LEDs and is switched using pulse width modulation (PWM) for luminous flux output control. This drive circuit has high power losses due to the biasing resistors and the biasing resistors can require costly calibration to provide accurate current control. In addition, U.S. Pat. No. 4,001,667 discloses a closed loop circuit that provides a train of constant current pulses to light-emitting diodes for luminous flux output control. This closed loop circuit however, does not enable full range current pulse duty factor control.
U.S. Pat. No. 6,586,890 discloses a method that uses a current feedback system to adjust power to LEDs in which a low frequency PWM control signal drives a power supply. This method however, uses PWM switching frequencies of 20 Hz to 20 kHz which generate audible noise and can detrimentally affect the LEDs by thermally cycling the LED dies which consequently reduces reliability and lifetime of the devices.
U.S. Pat. No. 6,734,639 discloses a method for limiting excess drive current transients of a switched drive circuit for LED arrays by means of a voltage converter combined with a customized sample-and-hold drive circuit. The LED drive signal is linked to a biased switching signal for repetitively turning ON and OFF the voltage converter to switch both the load and the power supply simultaneously. This method however, can only be applied to fly-back and push-pull type voltage converters and cannot control the LED drive current directly. This method does not significantly reduce drive circuit power loss or improve overall system efficiency. In addition, this method typically only works within desired parameters up to drive frequencies of the order of 400 Hz and does not allow for high frequency switching. Consequently, a drive circuit according to this method can generate undesirable audible noise and may impose excessive thermal stress on the connected LED arrays.
Moreover, U.S. Patent Application No. 2004/0036418 discloses a method for driving LED arrays in which a converter is used to vary the current through the LEDs. A current switch is implemented to provide feedback. This method incorporates essential elements of a standard Buck converter design, however it is not able to control parallel LED strings which require different forward voltages. This method discloses how to use high-side transistor switches as variable resistors to limit the current per LED string, however high-side transistor switches can induce large power losses and decrease the overall efficiency of the drive circuit.
Power Integrations Inc. provides analog integrated circuits which can effectively and efficiently control LEDs. Power Integrations have disclosed a power conversion technology called eDI-92 that only requires a minimum number of components and is particularly designed for low-energy consumption lighting applications, for example emergency exit or night light signs. However, this solution does not provide for dimming, a switching capability of the load, or a means for controlling the peak load current for instances where the voltage converter is switched.
Furthermore, austriamicroystems AG, offers a high performance analog integrated circuit AS3691 that can be used to control between one and four LEDs at drive currents of up to about 1.6 A in a single LED configuration and 400 mA for each LED in a four LED configuration. The AS3691 provides a very specific design of a voltage converter feedback circuit that can limit the output voltage of the voltage converter. This converter control chip however, does provide a means for maintaining a voltage adjustment for LED strings, with the possibility of digital switching for dimming. This chip uses internal current limiting over all duty cycles to ensure that the peak load current never exceeds a desired set point. This approach can result in lower overall system efficiency. This inefficiency may worsen at switching frequencies higher than a few hundred Hertz, as there is no provision for maintaining the voltage set point during the OFF period and therefore the internal current limiting circuitry would need to be active over most duty cycles. In addition this control chip does not allow for efficient drive of LEDs with a wide range of forward voltages, but typically requires to be tuned for each LED or string thereof by means of external resistors.
In addition, FIG. 1 illustrates a representation of the relative current that may flow through a load in a circuit in which the voltage converter is switched. The rise time 111 and fall time 112 of the current is directly related to the speed with which a switching voltage converter can change the current supplied to the load. For example, when this procedure is used for the activation of LEDs, the light output of the LED during the transition periods, for example rise and fall times, may not be at the desired level and therefore may result in variations in the light output, which may be readily apparent during low duty cycles, for example.
Therefore, there is a need for an apparatus and method for effective control and light-emitting element drive current electronic circuit design that overcomes problems identified in the prior art.
This background information is provided to reveal information believed by the applicant to be of possible relevance to the present invention. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art against the present invention.